hydroclimatic trends in the mississippi river basin from ... · milly and dunne (2001) deduced a...

Hydroclimatic Trends in the Mississippi River Basin from 1948 to 2004

TAOTAO QIAN, AIGUO DAI, AND KEVIN E. TRENBERTH

National Center for Atmospheric Research,* Boulder, Colorado

(Manuscript received 11 September 2006, in final form 18 January 2007)

ABSTRACT

The trends of the surface water and energy budget components in the Mississippi River basin from 1948to 2004 are investigated using a combination of hydrometeorological observations and observation-constrained simulations of the land surface conditions using the latest version of the Community LandModel version 3 (CLM3). The atmospheric forcing data for the CLM3 were constructed by adding theintramonthly variations from the 6-hourly National Centers for Environmental Prediction–National Centerfor Atmospheric Research (NCEP–NCAR) reanalysis to observation-based analyses of monthly precipi-tation, surface air temperature, and cloud cover. The model-based analysis suggests that, for the surfacewater budget, the observed increase in basin-averaged precipitation is compensated by increases in bothrunoff and evapotranspiration. For the surface energy budget, the decrease of net shortwave radiationassociated with observed increases in cloudiness is compensated by decreases in both net longwave radia-tion and sensible heat flux, while the latent heat flux increases in association with wetter soil conditions.Both the simulated surface water and energy budgets support the view that evapotranspiration has in-creased in the Mississippi River basin from 1948 to 2004. Sensitivity experiments show that the precipitationchange dominates the evapotranspiration trend, while the temperature and solar radiation changes haveonly small effects. Large spatial variations within the Mississippi River basin and the contiguous UnitedStates are also found. However, the increased evapotranspiration is ubiquitous despite spatial variations inhydrometeorology.

1. Introduction

Global surface temperatures have increased between0.4° and 0.8°C since the late nineteenth century(Houghton et al. 2001), and the associated changes inthe earth’s energy balance are likely to affect otherclimatic factors such as the components of the terres-trial water cycle (Huntington 2006; Margulis et al.2006). The partitioning of surface energy between sen-sible heating and evaporative cooling plays an impor-tant role in land–atmosphere interactions and climatechange (Koster et al. 2004; Seneviratne et al. 2006).Actual evaporation, or more generally evapotranspira-tion (referred to as E hereafter), is constrained at the

land surface by the energy and water budgets. The sim-plified land surface energy budget has five components:net shortwave or solar radiation, net longwave radia-tion, latent heat flux, sensible heat flux, and heat fluxinto the ground. The surface water budget over landincludes four components: precipitation, evapotranspi-ration, runoff, and the change of soil and groundwatercontent. Historical records generally exist only for thesolar radiation, precipitation, and runoff (discussed be-low), but not for the other components. In previousstudies, the incomplete observations of these two bud-gets have led to conflicting estimates of the trends inactual evapotranspiration over global land, includingthe United States (see review by Huntington 2006).

For the energy budget, observations show that overmost of the United States, total cloud cover has beenincreasing (Karl and Steurer 1990; Sun 2003; Groismanet al. 2004; Dai et al. 2006), leading to decreases insurface downward solar radiation (Gilgen et al. 1998;Liepert 2002). Another indirect indicator of increasedcloud and reduced surface solar radiation is the ob-served decrease in diurnal temperature range (DTR)over many land areas, including the United States from

* The National Center for Atmospheric Research is sponsoredby the National Science Foundation.

Corresponding author address: Dr. Taotao Qian, Climate andGlobal Dynamics Division, National Center for Atmospheric Re-search, P.O. Box 3000, Boulder, CO 80307.E-mail: [email protected]

15 SEPTEMBER 2007 Q I A N E T A L . 4599

DOI: 10.1175/JCLI4262.1

© 2007 American Meteorological Society

JCLI4262

1950 to the 1990s (Easterling et al. 1997), becauseclouds reduce surface solar heating and thus daytimemaximum temperature (Dai et al. 1999). There are nodirect observations of actual evapotranspiration exceptfor a few sites such as the Atmospheric RadiationMeasurement/Cloud and Radiation Testbeds (ARM/CART), the Oklahoma Mesonet, and some Russiansites with tower flux measurements or short-term fieldexperiments, and the FLUXNET measurements (Bal-docchi et al. 2001) only for the recent period, but anindirect observation—pan evaporation (Epan), which isthe evaporation from the open water surface of anevaporation pan—has shown decreasing trends fromthe 1950s or 1960s to the 1990s over the United Statesand parts of Europe and Siberia (Peterson et al. 1995),India (Chattopadhyay and Hulme 1997), and China(Thomas 2000; Liu et al. 2004). The decreases in panevaporation are physically consistent with decreasedsolar radiation (Roderick and Farquhar 2002) associ-ated with observed increases in cloud cover (Dai et al.1997a, 1999) or associated with increased aerosols andother air pollutants (Stanhill and Cohen 2001; Liu et al.2004) that have changed the optical properties of theatmosphere, in particular those of clouds. The surfacelatent heat flux is directly proportional to evapotrans-piration.

Based on the pan observations, Peterson et al. (1995)concluded that actual evapotranspiration over theUnited States and former Soviet Union may have de-creased from the 1950s to the 1990s because of reducedsurface heating. However, this is inconsistent with theobserved temperature increases in these areas. Basedon theoretical deductions, Brutsaert and Parlange(1998) concluded that actual evapotranspiration shouldbe negatively correlated with pan evaporation over re-gions with less than adequate moisture. Over northernextratropical land areas actual evapotranspiration mayhave increased due to increases in precipitation (e.g.,Dai et al. 1997b) and higher temperatures (e.g., Jonesand Moberg 2003), and thus the hydrological cycle mayhave intensified.

Further analyses suggest increasing trends in actualevapotranspiration over the United States and Russiaduring the period from the 1950s to the 1990s(Lawrimore and Peterson 2000; Golubev et al. 2001),and they support Brutsaert and Parlange’s (1998) inter-pretation. Using parallel observations of actual evapo-transpiration and pan evaporation at five Russian ex-perimental sites, Golubev et al. (2001) developed amethod to estimate actual evapotranspiration from panevaporation measurements and showed an actualevapotranspiration increase over most dry regions ofRussia from 1951 to 1987 and the United States from

1957 to 1998, and a humid region at the northwesterntip of the western United States from 1961 to 1998.Only over the heavily forested regions of Russia (1951–90) and the southeastern United States (1957–98) didthey find a decrease in actual evapotranspiration.

Based primarily on observations of net radiation overat least a decade from sparse locations over land (pre-dominantly over northern extratropical land), Wild etal. (2004) estimated that there has been a reduction innet radiative heating at the surface on the order of 1–28W m�2 century�1 between 1960 and 1990. Given theobserved surface warming, they inferred a decrease ofevaporative cooling over global land (assuming littlechange in sensible heating) during the same period.However, because sensible heat change is not negli-gible, their inference is not justified. Milly and Dunne(2001) deduced a large negative trend in the sensibleheat flux (–11 W m�2 century�1) over the Mississippibasin from 1949 to 1997 through semiempirical waterbalance theory and energy budget analyses.

Since evapotranspiration occurs mainly during mid-day with solar heating as the driving force, decreases insolar radiation could reduce evapotranspiration, al-though changes in soil moisture due to precipitationchanges may play a more important role over manyland areas. In other words, it is necessary to also con-sider the surface water budget to determine evapo-transpiration trends over most land areas.

For the water budget, observations show that precipi-tation increased over northern mid- and high-latitudeland areas during the twentieth century (e.g., Dai et al.1997b). Over most of the United States, historicalrecords show that both precipitation (P) and stream-flow or runoff (R) increased during the past 50–100 yr(e.g., Karl and Knight 1998; Groisman et al. 1999;Lettenmaier et al. 1999; Lins and Slack 1999; Kunkel etal. 1999; McCabe and Wolock 2002; Groisman et al.2001, 2004), and P � R has also increased (Milly andDunne 2001; Walter et al. 2004). Ignoring long-termchanges in land water storage, Walter et al. (2004) in-ferred, based on the P � R trend, that evapotranspira-tion increased over major U.S. river basins during thelast 50 yr. Based primarily on observed precipitationand streamflow and using mass balance, Milly andDunne (2001) deduced an increasing trend in actualevapotranspiration over the Mississippi River basinduring 1949–97.

In summary, although there is still some controversyabout the evapotranspiration trend (e.g., Ohmura andWild 2002; Wild et al. 2004), the weight of evidencesupports an increasing trend in evapotranspiration overthe United States. The controversy mainly arises fromincomplete observations of the energy and water bud-

4600 J O U R N A L O F C L I M A T E VOLUME 20

gets, which further hinders our understanding of thephysical mechanisms of evapotranspiration changes. Inthis study, a comprehensive evaluation of historicalchanges in these two budgets using available observa-tional data and observation-constrained land modelsimulations is made. Our goal is to provide a self-consistent analysis of the changes in surface water andenergy fluxes over the United States during 1948–2004,with a focus on the Mississippi River basin where long-term records of streamflow provide additional con-straints on the analysis. Because of incomplete obser-vations of surface water and energy fields, we employ acomprehensive land surface model, namely the Com-munity Land Model version 3 (CLM3), forced with ob-servation-based precipitation, temperature, and otheratmospheric forcing to simulate historical land surfaceconditions. The CLM3 simulations provide comple-mentary information for evapotranspiration, soil mois-ture, and surface energy fluxes that are physically con-sistent with available hydroclimatic records. They alsoprovide a means to examine sensitivities of surfaceevapotranspiration and other fluxes to individual atmo-spheric forcing (e.g., precipitation and temperature).Combining the CLM3 simulations with historical dataof temperature, precipitation, cloudiness, and stream-flow allows us to provide a full assessment of hydrocli-matic changes over the Mississippi River basin during1948–2004.

2. Data and analysis methods

Monthly observational data over the United Statesfrom 1948 to 2004 are available for surface air tempera-ture from the Climate Research Unit (CRU; Jones andMoberg 2003), precipitation (Chen et al. 2002), stream-flow (Dai and Trenberth 2002), cloud cover (Mitchell etal. 2004; Dai et al. 2006), and diurnal temperature range[from the Global Historical Climatology Network ver-sion 2 (GHCN2); Peterson and Vose 1997]. Relativelyshort records of surface solar radiation from a limitednumber of stations were available over the UnitedStates (Liepert 2002), but we used them only to evalu-ate the cloudiness-adjusted solar radiation (Qian et al.2006). Short records of soil moisture from Illinois werealso used only to evaluate the CLM3 simulations (Qianet al. 2006).

The historical records of monthly streamflow of theMississippi River basin are from gauge measurementsat Vicksburg, Mississippi (Dai and Trenberth 2002).The record extends only to September 1998. Themeasurements of gauge height were continually re-corded and are highly correlated with streamflow (r �0.96 for monthly data). The daily historical data ofgauge height (0800 CST reading) of 1947–2004 (from

http://rivergages.com/, which is maintained by the U.S.Army Corps of Engineers) were used. A linear regres-sion analysis of the monthly streamflow and gaugeheight for October 1947�September 1998 was per-formed and then used to derive monthly streamflowafter September 1998 from the gauge height data.

To complement the available observational data andfor sensitivity analyses, the CLM3 was used to simulatethe historical land surface conditions driven by obser-vation-constrained atmospheric forcing (Qian et al.2006). The CLM3 is a comprehensive land surfacemodel designed for use in coupled climate system mod-els (Dai et al. 2003; Oleson et al. 2004). The CLM3simulates all major land surface processes. They include1) vegetation composition, structure, and phenology; 2)absorption, reflection, and transmittance of solar radia-tion; 3) absorption and emission of longwave radiation;4) momentum, sensible heat, and latent heat (groundand canopy evaporation; plant transpiration) fluxes; 5)heat transfer in soil and snow; 6) canopy hydrology; 7)snow hydrology; 8) soil hydrology (surface runoff, in-filtration, subsurface drainage, and redistribution ofwater within the 10-layer column, which has a fixeddepth of 3.43 m); 9) stomatal physiology and photosyn-thesis; and 10) routing of surface runoff to the oceans.The CLM3 was run over global land from 1948 to 2004(after spinup, see Qian et al. for details) at T42 (�2.8°)resolution with the spatial heterogeneity of land surfacerepresented as a nested subgrid hierarchy in which gridcells are composed of multiple land units, snow/soil col-umns, and plant functional types.

The focus of this paper is the long-term (57-yr) trendof water and energy budgets averaged over the wholeMississippi River basin (2 896 000 km2), not the small-scale variability. For this purpose, we used the globalforcing dataset at T62 (�1.8°) resolution described byQian et al. (2006). It was derived by combining long-term variations from monthly time series of observedprecipitation, surface air temperature, and other cli-mate records with the short-term (intramonthly) varia-tions contained in the National Centers for Environ-mental Prediction–National Center for AtmosphericResearch (NCEP–NCAR) reanalysis (Kalnay et al.1996) for a 57-yr period (1948–2004) over global landareas. We also used the 40-yr European Centre for Me-dium-Range Weather Forecasts (ECMWF) Re-Analysis(ERA-40) data (1958–2001) for the intramonthly vari-ability. The results showed that the intramonthly vari-ability from both NCEP–NCAR and ERA-40 make nosignificant differences to the CLM3-simulated long-term trends. Although some long-term simulationshave used the NCEP–NCAR reanalysis data directly asatmospheric forcing (e.g., Fan et al. 2006), this was not


done here owing to spurious trends and biases (mainlybefore the 1970s). Instead, as given by Qian et al.(2006), the NCEP–NCAR reanalysis was used to onlyprovide the high-frequency information. Recently, theNorth American Regional Reanalysis (NARR; 1979–2003; Mesinger et al. 2006) was released and providesmuch higher spatial resolution and improved precipita-tion fields compared with the global NCEP–NCAR re-analysis. However it is not global, as is required for oursimulations. Also, its length of record is too short forlong-term trend analyses. This also applies to manyother model-assimilated land surface data, for example,the Global Soil Wetness Project (GSWP; Dirmeyer etal. 1999) and the Global Land Data Assimilation Sys-tem (GLDAS; Rodell et al. 2004). Further, some prob-lems have been identified in the NARR (e.g., in evapo-ration fields: see Nigam and Ruiz-Barradas 2006). Wehave performed test runs at 0.5° resolution for compari-son with those at T42 (�2.8°) with the CLM3 and re-sults show that large-scale (�500 km) variations arevery similar.

For observational precipitation, the Chen et al.(2002) dataset (for 1948–96) supplemented by theGPCP v2 data (Adler et al. 2003) for 1997–2004 waschosen because Chen et al. used a large network ofraingauges in creating the climatological mean fields,while the GPCP has better gauge coverage than theChen et al. data after 1997. Monthly surface air tem-peratures were derived by adding the temperatureanomalies from Jones and Moberg (2003) to the clima-tology of New et al. (1999). Surface downward solarradiation from the reanalysis was adjusted, first, forvariations and trends using gridded monthly cloudcover anomalies derived from surface observations(Mitchell et al. 2004) merged with the latest synopticobservations (see Dai et al. 2006) and, second, for meanbiases using satellite observations during July1983�June 2001. Because observational analyses ofsurface specific humidity for 1948–2004 are unavailableand the reanalysis relative humidity (RH) (constrainedby lower-tropospheric radiosonde data) is highly corre-lated with synoptic humidity observations from Dai(2006), we used the surface RH from the reanalysis toderive the specific humidity with the adjusted air tem-perature. Surface wind speed and air pressure were in-terpolated directly from the 6-hourly reanalysis data to3-hourly resolution (see Qian et al. 2006).

The CLM3 simulations were evaluated with availabledata on soil moisture and streamflow from the world’smajor rivers in Qian et al. (2006). The CLM3 success-fully reproduces the observed time series of river out-flow from most of the world’s top 200 rivers. This in-cludes the Mississippi River, with a correlation coeffi-

cient (r) of 0.94 (the mean bias is 96.1 km3 yr�1 or17.4% of the observed mean). It also captures the year-to-year variations in the few locations with observedsoil moisture content over Illinois (r � 0.73) and severalEurasian regions (r � 0.56 to 0.73).

To perform areal averaging and integration and toobtain estimates over drainage areas for individualriver basins, a database for the world river network isneeded. The simulated river database STN-30p (Vörös-marty et al. 2000), a global simulated topological net-work at 30� spatial resolution, was used. The observedrunoff for the Mississippi River basin was estimated bydividing the station streamflow at Vicksburg, Missis-sippi, by the drainage area upstream (2 896 000 km2).

Although the CLM3 simulations are on a T42 (about2.8°) Gaussian grid, the outputs were first regriddedonto a rectangular grid (2.5° � 2.5°) using bilinear in-terpolation and then to the 0.5° � 0.5° grid by simplyassigning all subgrid boxes to the value of the low-resolution box. The basin areal averaging is then per-formed on the 0.5° � 0.5° grid using the STN-30p riverdatabase. The annual means of all the variables in thisstudy are for the water year (1 October of the previousyear to 30 September of the year). This interval is usedbecause hydrological systems are typically at their low-est levels near 1 October in the Northern Hemisphere.

We analyze both the surface water and energy bud-gets. The simplified surface water budget over land canbe written as

dW�dt � P � E � R. �1�

The temporal change of land water storage dW/dt(which includes soil liquid and ice water, snow water,and canopy water) is equal to precipitation (P) minusevapotranspiration (E) and runoff (R) (which includesthe surface and subsurface flow). The human influencesthrough consumptive water use and filling of reservoirsand depletion of groundwater are discussed in Millyand Dunne (2001), and they are excluded in our analy-ses here. In the water budget, precipitation and runoffare from observations. The observation-constrainedCLM3-simulated evaporation, runoff, and change ofland water storage are used to complement the avail-able observational data.

The simplified surface energy budget can be written as

G � SW � LW � SH � LH, �2�

where these terms are the net shortwave (SW) andlongwave (LW) radiation, sensible heat (SH) and latentheat (LH) fluxes, plus the heat flux into the ground (G,including a small contribution due to snowmelt). In Eq.(2), LW, SH, and LH are positive upward whereas SWand G are positive downward. Atmospheric transports


may affect these quantities by altering surface air tem-perature and humidity. Except for downward short-wave (SW↓), which is derived from observed cloudcover, all other energy budget components are from theobservation-constrained CLM3 simulation. Trends inall time series data are analyzed using simple linearregression.

3. Results

a. Surface water budget

Figure 1 shows the time series of the annual mean ofthe terms in Eq. (1) based on either observations or the

observation-constrained CLM3 simulations for the Mis-sissippi basin, with the linear trends summarized inTable 1. From October 1948 to September 2004, thebasin-averaged precipitation (solid line) increased at anaverage rate of 85.5 7.4 mm century�1 or �11.3 1.0% century�1 (attained significance level p � 0.13).The observed runoff (solid line with triangles) has simi-larly increased at a rate of 67.6 4.8 mm century�1 or�35.3 2.5% century�1 (p � 0.07). The precipitationand runoff trends are compared with previous studies inTable 2. Karl and Knight (1998) reported a U.S. annualmean precipitation increase of 65–151 mm century�1,or 7.7%–19.5% century�1, for different periods anddifferent datasets. Groisman et al. (2004) updated U.S.

FIG. 1. Time series of annual (water year) surface water budget components averaged overthe Mississippi River basin and associated linear trends (straight lines: b is the slope in mmcentury�1). Given are the observed basin precipitation (P, solid line), runoff (Robs, solid linewith triangles), the water budget-derived evapotranspiration (EB, solid line with dots), andCLM3-simulated runoff (R, dashed line with triangles), evapotranspiration (E, dashed linewith dots), change of land water storage [dW/dt, solid line with quadrangles; note that b forthis quantity is d(dW/dt)/dt] and (bottom) land water storage (W, dashed line; b is the trendin W ).


annual precipitation trends for 1908–2002 and annualrunoff trends for 1939–2002, and found them to be 7%and 26% century�1, respectively. For the Mississippibasin, Walter et al. (2004) estimated increases of annualprecipitation of 176 mm century�1 and for streamflowof 65 mm century�1 from 1950 to 2000. Although theselinear trends differ somewhat, they are all for slightlydifferent time periods, and our precipitation andstreamflow trends are generally consistent with them.

Substantial biases in rain gauge records of precipita-tion exist because of undercatch (especially for snow)due to winds and evaporation within the gauge. Yang etal. (2005) adjusted daily precipitation records from4802 high-latitude stations to correct for the biases ofwind-induced undercatch, wetting losses, and trace pre-cipitation amounts for the last 30 yr. Bias correctionsgenerally enhance monthly precipitation trends by 5%–20%. In this study the precipitation data were not cor-rected for undercatch and other biases over the UnitedStates. The observed streamflow is affected by humanwithdrawals and water management. Although waterresource development has played only a small role inthe time-mean water balance, Milly and Dunne (2001)estimated that irrigation and other human activities

(consumptive water use) may have contributed about26 mm century�1 (or �25%) to the increasing trend intotal evaporation over the Mississippi basin, while thestorage rates associated with filling of surface reservoirsand long-term depletion of groundwater were esti-mated as 5 and 3 mm century�1, respectively. Gedneyet al. (2006) recently suggested that the increasing run-off trends are consistent with a suppression of planttranspiration due to CO2-induced stomatal closure.

The CLM3-simulated runoff (Fig. 1, dashed line withtriangles) increased at a rate of 55.9 5.6 mm cen-tury�1, or about 21.7 2.2% century�1 (relative to thesimulated mean, p � 0.19). It is highly correlated withthe observed annual runoff (r � 0.90, with mean bias of66.3 mm or 34.6% of the observed mean). The CLM3-simulated evapotranspiration (dashed line with dots)increased at a rate of 32.4 2.7 mm century�1, or about6.5 0.5% century�1 (p � 0.11). Although the CLM3-simulated land water storage W (Fig. 1, bottom) in-creased at a rate of 18.0 mm century�1 (p 0.01), thetrend in the change of water storage [dW/dt, its trend isd(dW/dt)/dt], which is dominated by changes in soil wa-ter, decreased at a very small and insignificant rate of2.8 mm century�1. The water budget–derived evapo-

TABLE 2. Annual precipitation and runoff trends from different datasets and different periods.

Studies RegionPeriod of

precipitationPrecipitation trend

(mm century�1)/(% century�1)Period of

runoffRunoff trend

(mm century�1)/(% century�1)

Karl and Knight (1998)HCNs United States 1910–96 81/10.1CD United States 1910–96 76/10.0CD United States 1910–96 65/7.7TD 3200 United States 1948–95 128/16.9HCNs United States 1948–95 110/14.7CD United States 1948–95 151/19.5Groisman et al. (2004) United States 1908–2002 53/7 1939–2002 N/A/26Walter et al. (2004) Mississippi 1950–2000 176/N/A 1950–2000 65/N/APresent study Mississippi 1948–2004 85.5/11 1948–2004 68/35

Mississippi 1950–2000 171/23 1950–2000 81/31

HCNs: Historical Climate Network special network.CD: the U.S. climatological division dataset.

TABLE 1. Annual (water year) trends from October 1948 to September 2004 of the water budget components averaged over theMississippi basin as shown in Fig. 1.

VariablesBasin

mean (mm)Slope b

(mm century�1)Standard error of b

(mm century�1) p value Correlation

Precipitation 756 85.5 7.4 0.13Runoff 258 55.9 5.6 0.19 0.90 (vs Robs)Evapotrannspiration 499 32.4 2.7 0.11 0.82 (vs EB)dW/dt 0 �2.8 1.2 0.75Robs 191 67.6 4.8 0.07EB 565 20.7 4.2 0.51


transpiration (EB, where B is budget; solid line withdots), as the residual of precipitation, observed runoff,and CLM3-simulated change of water storage, also in-creased at a rate of 20.7 4.2 mm century�1, or about3.7 0.7% century�1 (p � 0.51). It is correlated withthe model-simulated evapotranspiration (r � 0.82),which has a larger increase of 32.4 mm century�1.Therefore, in terms of linear trends, the increase ofprecipitation is compensated by both runoff (65%) andevapotranspiration (38%, based on EB), plus a smallloss of change of water storage (as shown schematicallylater in Fig. 6). The increasing trend is consistent withthe results of previous mass balance studies (Milly andDunne 2001; Walter et al. 2004). As the contributionfrom change of water storage is about 3%, this study

actually adds credence to earlier mass balance ap-proaches in which they assumed that internal net waterstorage is negligible.

b. Temperature, cloud, and diurnal temperaturerange

The Mississippi River basin-averaged temperaturefrom the CRU dataset increased linearly at a rate of0.9° 0.1°C century�1 (p � 0.05) from 1948 to 2004(Fig. 2, top, solid line, Table 3). The temperature fromthe GHCN2 homogeneity-adjusted data (dashed line)also increased, at a similar rate of 1.0° 0.1°C cen-tury�1 (p � 0.03). These values agree within the stan-dard error of slope b (Table 3). Linear trends are quite

FIG. 2. Anomaly time series of observed annual (water year) (top) surface air temperaturefrom the CRU dataset (T2m_cru, solid line) and the GHCN2 dataset (T2m_ghcn, short dashedline), and (bottom) cloud cover (CLD, solid line), and diurnal temperature range [DTR, longdashed line; scaled as –5DTR�5, from Dai et al. (2006)], and cloud-adjusted downward solarradiation (SW↓, short dashed line; increases downward on the right ordinate) in the averagedover the Mississippi River basin and associated linear trends (straight lines: b is the slope, °Ccentury�1 for T and DTR, % century�1 for CLD, and W m�2 century�1 for SW↓).


a lot less (0.3°C century�1) for 1949–1997, which wasthe period analyzed by Milly and Dunne (2001) usingthe original uncorrected station data (P. C. D. Milly2006, personal communications), but station tempera-ture data from many U.S. stations contain large noncli-matic changes due to instrumental changes (Petersonand Vose 1997), and the adjustments in GHCN2 re-move most of these nonclimatic changes. Thus the ho-mogeneity-adjusted GHCN2 data used here are likelymore reliable than Milly and Dunne (2001).

The DTR from the GHCN2 shows an overall lineardecreasing trend of 0.7 0.1°C century�1 (p � 0.06)from 1948 to 2004 over the Mississippi River basin (Fig.2, bottom, long dashed line, scaled as –5DTR�5). TheDTR decreases, together with the upward trend in pre-cipitation (Fig. 1), are physically consistent with theincreasing trend of 9.0 0.3% century�1 of sky (p 0.01) in observed total cloud cover from 1948 to 2004(solid line), as clouds have a dominant damping effecton the DTR (Dai et al. 1999). Correspondingly, thecloud-adjusted downward solar radiation (short dashedline, right-hand ordinate) decreased at a rate of 10.2 0.3 W m�2 century�1 (p 0.01). Increasing anthropo-genic aerosol concentrations (Stanhill and Cohen2001), the extinction effects of volcanic eruptions(Schwartz 2005), or the population/urbanization ef-fect (Alpert et al. 2005) may contribute to solar dim-ming in some locations. Whether the increased cloudcover is due to these factors cannot, however, be ascer-tained.

c. Surface energy budget

Figure 3 shows the 1948–2004 time series of the an-nual anomalies for the surface energy budget compo-nents. The downward shortwave flux SW↓ (thick solidline with dots) decreased by 10.2 0.3 W m�2 cen-tury�1, and the reflected upward shortwave flux SW↑(thick dashed line with triangles) decreased by 4.6 0.1W m�2 century�1. This implies that the net shortwaveflux Fsw (positive downward) decreased by 5.7 0.3 W

m�2 century�1, or about 4.3 0.2% century�1 (p �0.01; Table 4).

Figure 3 shows that the downward longwave fluxLW↓ (thin solid line with dots) increased by 11.6 0.3W m�2 century�1, and the emitted upward longwaveflux LW↑ (thin dashed line with triangles) increased by5.3 0.3 W m�2 century�1. This implies that the netlongwave flux Flw (positive upward) decreased, at a rateof 6.3 0.2 W m�2 century�1, or 9.9 0.3% century�1

(p 0.01). Therefore, the atmospheric longwave radia-tion increases have largely offset solar radiation de-creases. This corroborates the earlier hypothesis byWalter et al. (2004).

Although both ground and air temperatures in-creased from 1948 to 2004, the increasing rate forground temperature is less than that for surface air tem-perature (not shown). These are consistent with thechanges in net longwave radiation in which LW↓ in-creased by 11.6 W m�2 century�1 while LW↑ increasedby only 5.3 W m�2 century�1. Also, as a result, thesurface sensible heat flux decreased by 1.8 0.3 Wm�2, or 6.5 1.1% century�1 (p � 0.43) (Fig. 3, longdashed line), while the latent heat flux (short dashedline) increased at a rate of 2.5 0.2 W m�2, or about 6.3 0.5% century�1 (p � 0.12). Note that sensible andlatent heat fluxes vary inversely (r � �0.63), as ex-pected from the repartitioning of energy with changesin surface moisture (LeMone et al. 2003; Trenberth andShea 2005).

The residual (heat flux into soil, bottom line in Fig. 3)is very small. Therefore, in terms of long-term changes,there are strong compensations between net shortwaveand longwave radiation (r � 0.65) and between sensibleand latent heat fluxes (r � �0.63). Hence the change innet total radiation is small. Overall, the decreases in netlongwave radiation and sensible heat flux not only off-set the reduction in shortwave radiation but also allowan increase in latent heat flux. This is depicted sche-matically in Fig. 6, described later. Therefore, the sur-face heat balance is a four-way balance, and the in-crease of latent heat flux is not caused by the change of

TABLE 3. Annual (water year) trends from October 1948 to September 2004 of 2-m air temperature, total cloud cover, DTR, anddownward shortwave radiation averaged over the Mississippi Basin, as shown in Fig. 2.

Variables Basin mean Slope bStandarderror of b b units p value Correlation

T2m_ghcn 11.0 (°C) 1.0 0.1 (°C century�1) 0.03T2m_cru 10.2 (°C) 0.9 0.1 (°C century�1) 0.05Cloud 9.0 0.3 (% century�1) 0.01 0.62 (vs P)DTR �0.7 0.1 (°C century�1) 0.06 �0.72 (vs cloud)SW↓ 178 (Wm�2) 10.2 0.3 (W m�2 century�1) 0.01


shortwave flux. This does not support the assumed two-way balance between latent heat flux and shortwaveflux in Roderick and Farquhar (2002) or the incompletebalance in Wild et al. (2004).

d. Sensitivity experiments on factors affectingevapotranspiration trends

While isolating one factor and fixing other factors isidealistic, it can be useful to see how linear the differentfactors are. For example, increases in precipitation areusually associated with decreases in air temperatureand increases in cloud amount, and increases of cloudamount are associated with decreases of downwardshortwave radiation and increases of downward long-wave radiation. However, such idealized experimentsdo help us understand the dominant factors affectingthe final evapotranspiration trend.

Figure 4 shows the results from sensitivity experi-ments that explore factors affecting the evapotranspi-ration trend. The solid line is from the standard run(same as the dashed line with dots in Fig. 1). The shortdashed line is from a run forced with varying precipi-tation but fixed other forcing [i.e., other forcings wereset to the data of a specific year (1979) for 1948–2004;dP run]. The resultant E trend is 55.6 mm century�1 (p� 0.05), larger than the trend of 32.4 mm century�1 (p� 0.11) in the standard (all forcing) run. The longdashed line is from a run with only temperature forcing(others fixed, dT run). The trend in this run is 3.6 mmcentury�1 (p � 0.45), much smaller than that in thestandard run. The bottom line in Fig. 4 is from a runwith only downward solar radiation (i.e., cloud) forcing(dSW↓ run), and it has a negative trend of �23.5 mmper century (p 0.01). This is expected, as decreasing

FIG. 3. Annual (water year) anomaly time series of surface energy budget componentsaveraged over the Mississippi River basin and associated linear trends (straight lines: b is theslope, in W m�2 century�1). Except for downward shortwave (SW↓, thick solid line withdots), which is derived from observed cloud cover, all other components are from the obser-vation-constrained CLM3 simulation: upward shortwave (SW↑, thick dashed line with tri-angles), downward longwave (LW↓, thin solid line with dots), upward longwave (LW↑, thindashed line with triangles), latent heat flux (LH, short dashed line), sensible heat flux (SH,long dashed line), and heat flux into the ground (G, bottom line).


solar radiation reduces surface energy available forevapotranspiration if there are no other changes. Inaddition, we have calculated the correlation betweenthe simulated evapotranspiration and the forcing vari-

ables. The correlation with precipitation is 0.72, withtemperature is �0.01, and with downward shortwaveradiation is �0.53. Therefore, these results suggest thatthe increase in evapotranspiration from 1948 to 2004

FIG. 4. Anomaly time series of annual (water year) surface evapotranspiration averagedover the Mississippi River basin and associated linear trends (straight line: b is the slope, inmm century�1) from CLM3 sensitivity experiments: Standard � all forcing run (solid line); dPonly � with only precipitation changing and other forcings fixed (short dashed line); dTonly � run with only temperature changing (long dashed line); dSW↓ only � with onlydownward solar radiation (i.e., cloud) changing (bottom line).

TABLE 4. Annual (water year) trends from October 1948 to September 2004 of the energy budget components averaged over theMississippi Basin, as shown in Fig. 3.

VariablesBasin mean

(W m�2)Slope b

(W m�2 century�1)Standard error of b(W m�2 century�1) p value Correlation

SW↓ 178 �10.2 0.3 0.01SW↑ 47 �4.6 0.1 0.01Fsw 131 �5.7 0.3 0.01LW↓ 309 11.6 0.3 0.01LW↑ 372 5.3 0.3 0.04Flw 63 �6.3 0.2 0.01 0.65 (vs Fsw)LH 40 2.5 0.2 0.12SH 28 �1.8 0.3 0.43 �0.63 (vs LH)G 1 �0.1 0.0 0.82


was induced predominantly by the increase in precipi-tation, and partly offset by cloud-induced reduction insolar radiation.

e. Spatial distributions of trends

Figures 5a–d show the linear trend maps of the waterbudget terms over the contiguous United States. Thestippled area is the Mississippi River basin with thedark line as the boundary. Precipitation (Fig. 5a) in-creased over the entire basin during 1948–2004 with thelargest increases in the central part of the basin. Overthe central and eastern part of the basin, CLM3-simulated runoff (Fig. 5b) increased while trends inevapotranspiration (Fig. 5c) are relatively small. Overthe western part of the basin, evapotranspiration in-creased while trends in runoff are small. Trends inchange of land water storage are very small over mostof the basin (Fig. 5d).

The trend pattern in evapotranspiration is deter-mined by several factors. Over the western part of thebasin, surface soil and air are relatively dry (Dai 2006),runoff is limited, and water is a limiting factor forevapotranspiration, whereas atmospheric demand, asgiven through temperature and energy supply, is im-portant over the relatively wet central and eastern partof the basin. Consequently, trends in evapotranspira-tion in the western part follow more closely the pre-cipitation changes even though cloud-inferred down-ward solar radiation decreased (Figs. 5f–g), whereas de-creasing solar radiation over the eastern part largelyoffsets the positive impact of increasing precipitationon evapotranspiration.

Surface air temperature (Fig. 5e) shows large in-creases (0.8°–2.4°C century�1) during 1948–2004 in thenorthern and western part of the Mississippi basin,while little change or small cooling is seen in the south-east of the basin, consistent with the results of Grois-man et al. (2004). Total cloud cover (Fig. 5f) increasedover most of the basin by about 4%–12% of sky cen-tury�1 and is the main cause for the reduced surface netshortwave radiation shown in Fig. 5g.

The trend maps of the energy budget terms during1948–2004 (Figs. 5g–j) show that the decreasing trendpattern in net shortwave radiation (Fig. 5g) (by about2–12 W m�2 century�1 over the Mississippi basin) gen-erally follows the cloudiness change map (Fig. 5f). Thenet longwave flux (Fig. 5i) decreased by 4–12 W m�2

century�1 over most of the Mississippi basin, althoughthe trend in the western half is about twice as large asin the eastern half. Surface latent heat flux (Fig. 5h),which is essentially surface evapotranspiration (Fig. 5c),increased (by 2–12 W m�2 century�1) over the centraland western United States, but decreased (by 2–4 W

m�2 century�1) in the eastern part of the country. Sur-face sensible heat flux (Fig. 5j) increased (by 2–8 Wm�2 century�1) in the northwestern part of the basin,but decreased (by 2–8 W m�2 century�1) in the otherparts of the basin. Therefore, the strong compensationbetween short- and longwave radiation occurs on re-gional and local scales (Fig. 5k). The net radiation in-creased (by 2–6 W m�2 century�1) in the western partof the basin, but decreased (by 2–6 W m�2 century�1)in the eastern part of the basin. Similarly, there is strongcompensation between sensible and latent heat fluxeson regional and local scales (Fig. 5l). Thus, the largedecrease in the net longwave radiation (positive up-ward) not only offsets the reduced solar radiation butfurther provides energy for enhanced evapotranspira-tion (i.e., increased LH flux) over the western and east-ern United States; whereas in the eastern part, reducedSH flux also contributes to balance the decrease in solarradiation. Over the Mississippi basin, sensible heat fluxalso plays an important role in balancing the surfaceenergy budget.

4. Summary and discussion

Changes in the surface water and energy budget com-ponents over the Mississippi River basin during 1948–2004 have been investigated using a combination ofhydroclimatic records and observation-constrainedsimulations of the land surface conditions using a com-prehensive land surface model (namely, the CLM3).The atmospheric forcing for driving the CLM3 wereconstructed by combining observed monthly precipita-tion, temperature, and cloud cover with intramonthlyvariations derived from the 6-hourly NCEP–NCAR re-analysis.

In the offline observation-constrained CLM3 runs,the atmospheric downward longwave radiation is derivedfrom the surface air temperature and vapor pressurewithout considering clouds. Since lower-troposphericair temperature and water vapor control downwardlongwave radiation at the surface with only small ef-fects from clouds outside the polar regions (Zhang et al.1995), the longwave fluxes from the CLM3 simulationsmay be considered as a first-order estimate. We did notuse the NCEP–NCAR reanalysis downward longwaveradiation because its air temperature and cloud trendsare problematic. Although the NARR surface air tem-perature trend over the Mississippi Basin is quite con-sistent with the observation, the total cloud cover showsa decreasing trend of �4.8% century�1 from 1979 to2004, which is opposite to the increasing trend fromobservation (16.6% century�1). Reliable records of sur-face longwave radiation measurements are generally


FIG. 5. Spatial distributions of the linear trend from October 1948 to September 2004 in annual (water year) (a) observed precipi-tation; CLM3-simulated (b) runoff, (c) evapotranspiration, and (d) change of water storage; (e) observed surface air temperature; (f)observed cloud amount; and CLM3-simulated (g) net shortwave radiation, (h) latent heat flux, (i) net longwave radiation, (j) sensibleheat flux, (k) net radiative heating, and (l) net turbulent flux. The stippled area is the Mississippi basin with the dark line as theboundary.


Fig 5 live 4/C

unavailable prior to the 1990s. Philipona et al. (2004)found a significant increase in downward longwave ra-diation observed at several stations in the Swiss Alpsbetween 1995 and 2002. As clouds affect surface down-ward longwave radiation by changing atmosphericemissivity, Crawford and Duchon (1999) showed thatincreasing clouds imply increased downward surfacelongwave radiation. We infer that, if clouds had beenincluded in the offline CLM3 downward longwave ra-diation calculation, the net longwave flux would be de-creasing at a larger rate, which would further mitigatethe effect of reduced solar radiation on evapotranspi-ration. Philipona and Dürr (2004) showed that in-creases in surface net radiative heating over central Eu-rope for the periods 1995–2003 were dominated by theclear-sky longwave radiation component resulting froman enhanced water vapor greenhouse effect, while theeffects of changing cloud amounts on downward solarradiation and downward longwave radiation compen-sated each other and had little effect on annual-meansurface warming.

To a surprising extent, the main changes in both wa-ter and energy budget components for 1948 to 2004 canbe characterized by linear trends that seem to be physi-cal (real, not bad data) and statistically significant, andthe trends in different variables are physically consis-tent with one another. This justifies our focus on lineartrends, part of which is a posteriori reasoning.

The results of basin-averaged linear trends in waterand energy budgets during 1948–2004 are summarizedschematically in Fig. 6 in which the downward arrowmeans that the flux increases the trend of dW/dt or G.For the surface water budget, the increase in observedbasin-averaged precipitation (85.5 mm century�1) iscompensated by the increases in both observed runoff(67.5 mm century�1) and inferred evapotranspiration(20.7 mm century�1), with only a small decrease in thechange of land water storage (2.8 mm century�1). Forthe surface energy budget, the decrease of net solarradiation (5.7 W m�2 century�1) associated with theobserved increase in cloud cover (9.0% sky century�1)is compensated by a decrease of net longwave radiation(6.3 W m�2 century�1) and sensible heat flux (1.8 Wm�2 century�1), while the latent heat flux increases (by2.5 W m�2 century�1) in association with increased pre-cipitation and enhanced longwave radiative heating.Our results of both the surface water and energy bud-gets support the notion that evapotranspiration has in-creased over the Mississippi River basin during 1948–2004, although the basin-averaged evapotranspirationand latent heat flux trends are not statistically signifi-cant. Sensitivity experiments suggest that the evapo-transpiration trend mainly results from the precipita-

tion change, with smaller effects from the temperatureand solar radiation changes.

Our results also show that the change in surface sen-sible heat flux is comparable with changes in the otherenergy components. Thus it should be included in theenergy budget analysis, although it has often been ig-nored (Wild et al. 2004). The surface net radiative heat-ing (Fsw–Flw, positive downward) increased by 0.6 Wm�2 century�1, which is only one-third of the change insensible heat flux (1.8 W m�2 century�1). As mentionedin the introduction, many previous studies consideredonly the change in downward solar radiation (i.e., dim-ming), which is insufficient for assessing the change inthe energy budget.

However, it is evident that changes are not uniformacross the basin. Observed precipitation increases inthe central and eastern parts of the Mississippi basin arebalanced mainly by increased runoff, with small

FIG. 6. Basin-averaged trends in the water and energy budgetcomponents for the Mississippi River basin: M is the long-term(1948–2004) annual (water year) mean (in mm for water compo-nents and W m�2 for energy components) and b is the annuallinear trend during 1948–2004 (in mm century�1 for water com-ponents and W m�2 century�1 for energy components, propor-tional to arrow shaft width). Note that the downward arrow meansthat the flux increases the trend of dW/dt or G.


changes in evapotranspiration; while over the westernpart of the basin increased precipitation results in en-hanced evapotranspiration but little change in runoff.The decrease of the net shortwave radiation is mainlycompensated by the reduction in upward net longwaveradiation in the western part of the basin, but also withcontributions from reduced sensible heat flux in theeastern part of the basin. The spatial patterns of thetrends are consistent with the fact that in water-limitedenvironments (such as the arid western United States)surface evapotranspiration is mainly controlled byavailable water (Hobbins et al. 2004), whereas over therelatively wet central and eastern United States, avail-able energy plays an important role in regulating sur-face evapotranspiration.

This study also has several limitations, including un-certainties in model forcing data and deficiencies in theCLM3. The uncertainties in the forcing data includeundercatch and topography-induced biases in precipi-tation, the adjustment of the solar radiation using theoften incomplete cloudiness data, and a lack of realisticaccounting of LW–cloud interaction. The CLM3 mayhave been tuned to unrealistic high frequency and lowintensity of precipitation (Dai and Trenberth 2004),and it needs improved representations of land hydrol-ogy such as the partitioning of evapotranspiration (Wuand Dickinson 2005; Dickinson et al. 2006), as well astreatment of human influences on natural streamflowand groundwater. Currently a bunch of improvementsare ongoing, for example, on the partitioning of evapo-transpiration (Lawrence et al. 2007), soil water avail-ability, and soil evaporation (K. W. Oleson et al. 2007,personal communication), surface and subsurface run-off (Niu et al. 2005), and groundwater and water tabledepth (Niu et al. 2007). Future improvements in theseaspects are expected to decrease the uncertainties inthe estimated hydroclimatic trends.

Acknowledgments. This work was supported by NSFGrant ATM-0233568 and NCAR TIIMES WaterCycles across Scales Project. The first author would liketo thank Dennis Shea and Adam Phillips for their helpwith the NCL scripts. We also thank Andy Pitman andthree anonymous reviewers for their thorough and per-ceptive reviews that did much to improve the manu-script.

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